The conversion of existing natural gas storage caverns for hydrogen storage requires the assessment of the mechanical integrity of the wellbore. The additional impact on the wellbore induced by cavern convergence due to long-term operation must be particularly considered. Numerical simulations of an existing cavern and its wellbore using the FTK-simulator are performed to investigate the stress and strain changes in the casing shoe, including casing, annulus cement, and contact interfaces during the operation. In addition, hydrogen transport in the casing shoe area at the borehole contour is numerically simulated during the mechanical integrity test. It is shown that the FTK-simulator can be successfully used to predict the TH2M-couped load-bearing behavior of salt cavern and wellbore throughout its entire life and provides a detailed insight into the development of stress and strain regarding the casing, annulus cement, and contact interfaces.
According to LBEG (2022), there are 273 operating natural gas storage caverns in Germany with a maximum working gas storage capacity of about 14.8 billion m3, about 14% of Germany's total annual natural gas consumption. With further decarbonization in Germany, the number of caverns for natural gas storage could gradually decrease. Therefore, existing natural gas storage caverns can be further converted for green hydrogen storage. In this context, assessing the existing caverns, including their wellbores, is particularly important for conversion and long-term safe operation. Part of the assessment is to evaluate the mechanical integrity of the wellbore in addition to the geomechanical condition of the storage cavern, especially after many years of cavern operation.
In the casing shoe area, the primary barrier, consisting of the last cemented casing, the annulus cement, and the rock salt, is an essential part of the wellbore integrity. The lowest part of wellbore, the so-called casing shoe area is in direct contact with natural gas or hydrogen and must be safe to the extent necessary to seal the cavern. However, this part of the wellbore is most affected by the cavern. The cavern convergence induced by the operation leads to additional stresses in the casing shoe area, which are not considered in the design up to now. BVEG (2017) requires that the casing and annulus cement must withstand all possible loads during the lifetime of the cavern system. Numerical simulations on the effects of additional stresses due to cavern convergence have been performed in several papers (e.g. Park et al. 2006 and Orlic et al. 2016). Lux et al. (2020) have developed a numerical simulator that can explicitly simulate the cavern and the wellbore, particularly in the casing shoe area, including casing, annulus cement, and interfaces after long year cavern operation. Subsequently, gas transport simulation and the well integrity analysis can be performed.
